1. Field of the Invention
The present invention relates to an electronic control apparatus for an air-bag system, especially to an electronic control apparatus for an air-bag system with a capability of diagnosing accurately whether or not a back-up power is normal.
2. Description of the Related Art
In recent years, air-bag systems have been widely used for protecting passengers in a crash. As it is necessary for an air-bag to definitely expand when a crash occurs, but never to expand under normal circumstances, an electronic control apparatus using a microcomputer system is generally used as a control apparatus for the air-bag system. In the system, a capacitor having a large capacitance is provided to function as a back-up power source in the event that a battery that normally provides power is failed. The back-up capacitor is normally charged by the battery, and the electronic control apparatus is powered by the back-up capacitor if the battery is failed.
FIG. 6 shows a functional diagram of a conventional electronic control apparatus for an air-bag system. In the figure, a controller 63 is connected to the plus terminal of a battery 61 through an ignition switch 62. A back-up capacitor 64 is connected in parallel to the controller 63 through a diode 65 and charging resistor 66 which are connected in parallel with each other. When the battery 61 is operating normally, it supplies electric power to the controller 63 through the ignition switch 62, and charges the back-up capacitor 64 through the charging resistor 66. The direction of current flow is indicated by a solid arrow (a). If the terminal voltage Vb of the battery 61 drops or the connection is somehow broken, the charge stored in the back-up capacitor 64 is used to supply power to the controller 63 through the diode 65 to ensure minimal functioning. The direction of this current flow is indicated by a dotted arrow (b).
The controller 63 incorporates a diagnosing routine which diagnoses whether or not the back-up capacitor 64 is normal. This routine is executed each time the ignition switch 62 is turned on.
The conventional diagnosing scheme is to monitor the charge time of the capacitor to reach a voltage Vc, and judge that the back-up capacitor 64 is normal if the time required from the beginning of the diagnosis when the capacitor voltage Vc is at minimum, to the end of the diagnosis when Vc reaches its maximum value, is within a predetermined time interval.
If the back-up capacitor 64 becomes open, for example, if the lead wire 67 becomes disconnected, the capacitor voltage Vc reaches the terminal voltage Vb of the battery 61 immediately after the ignition switch 62 is turned on. Therefore, this kind of failure is detectable. If the back-up capacitor 64 becomes shorted, the capacitor voltage Vc does not increase after the ignition switch 62 is turned on. Therefore, this kind of failure is also detectable.
The controller 63, however, may spontaneously begin its diagnosis if it is reset by an extraneous noise, since it cannot determine whether a reset signal comes from the ignition switch 62 or is merely an extraneous noise. However, since discharging of the back-up capacitor 64 is not caused by an extraneous noise, the capacitor voltage Vc remains at the battery voltage Vb, so the diagnosis erroneously detects an open failure.